The present invention is directed to data signal demodulators and, more particularly, to novel frequency-shift-keyed data demodulators having an easily varied operating frequency range.
Digital data is commonly sent over a communications channel by shifting the frequency of a transmitted communications signal between discrete values representing the digital information. For binary digital data, the carrier frequency is typically shifted to a higher frequency for transmitting a first binary value and to a lower frequency for transmitting the remaining binary value. A data receiver receives the frequency-shift-keyed (FSK) carrier and decides whether the higher or lower frequency is being received, to provide a data output having the first or second binary value. While many FSK demodulator circuits are known to the prior art, not all of these circuits are capable of being monolithically integrated into a single, low cost integrated circuit component. In many integratable FSK demodulators, it is difficult, if not impossible, to shift the demodulator operating frequency band; it is, however, highly desirable to be able to shift this operating frequency band by merely changing a reference frequency. The ability to shift the operating frequency band by changing a reference frequency is particularly desirable for use in a data receiver in a frequency-hopped spread spectrum data transmission system.